The present invention discloses methods for enhancing the wear-resistance of polymers, the resulting polymers, and in vivo implants made from such polymers. One aspect of this invention presents a method whereby a polymer is irradiated, preferably with gamma radiation, then thermally treated, such as by remelting of annealing. The resulting polymeric composition preferably has its most oxidized surface layer removed. Another aspect of the invention presents a general method for optimizing the wear resistance and desirable physical and/or chemical properties of a polymer by crosslinking and thermally treating it. The resulting polymeric compositions is wear-resistant and may be fabricated into an in vivo implant.
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1. A preformed polymeric composition comprising a polymer that is crosslinked without melting or annealing it, the crosslinked polymer is subsequently melted in ambient atmosphere, and the melted polymer is subsequently cooled to room temperature, and the most oxidized surface of the cooled polymeric composition is removed, said preformed polymeric composition is characterized by the following characteristics: a degree of swelling of between about 1.7 and about 5.3; a molecular weight between crosslinks of between about 400 and about 8400 g/mol; and a gel content of between about 95% and about 99%.
5. A method for increasing the wear resistance of a preformed polymeric composition, comprising the steps of:
(a) crosslinking the polymeric composition without melting or annealing it;
(b) subsequently melting and then cooling the crosslinked polymeric composition to room temperature in ambient atmosphere to produce a polymeric composition that possesses the following characteristics: a degree of swelling of between about 1.7 and about 5.3; a molecular weight between crosslinks of between about 400 and about 8400 g/mol; and a gel content of between about 95% and about 99%; and
(c) removing the most oxidized surface of the crosslinked and subsequently melted and then cooled polymeric composition.
2. The preformed polymeric composition of
3. The preformed polymeric composition of
6. The method of
7. The method of
8. The method of
9. The method of
10. The method of
11. The method of
13. The method of
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This is a continuation of co-pending U.S. patent application Ser. No. 10/936,149, filed on Sep. 7, 2004, which is a continuation of Ser. No. 09/795,229, filed on Feb. 26, 2001, entitled “CROSSLINKING OF POLYETHYLENE FOR LOW WEAR USING RADIATION AND THERMAL TREATMENTS”, and issued as U.S. Pat. No. 6,800,670 on Oct. 5, 2004, which is a continuation of application Ser. No. 09/214,586, filed on Jan. 6, 1999, and issued as U.S. Pat. No. 6,228,900, on May 8, 2001, which is the national phase filing of Patent Cooperation Treaty application number PCT/US97/11947, filed on Jul. 8, 1997, which is based on U.S. provisional applications: Ser. No. 60/017,852 filed on Jul. 9, 1996; Ser. No. 60/025,712 filed on Sep. 10, 1996; and Ser. No. 60/044,390, filed on Apr. 29, 1997. The entire contents of the predecessor applications are herein expressly incorporated by reference.
The present invention relates to polymers. It discloses methods for enhancing the wear-resistance of polymers by crosslinking and thermally treating them. The polymers disclosed herein are useful for making implants, for example, as components of artificial joints such as acetabular cups.
Ultrahigh molecular weight polyethylene (hereinafter referred to as “UHMWPE”) is commonly used to make prosthetic joints such as artificial hip joints. In recent years, it has become increasingly apparent that tissue necrosis and interface osteolysis, in response to UHMWPE wear debris, are primary contributors to the long-term loosening failure of prosthetic joints. For example, wear of acetabular cups of UHMWPE in artificial hip joints introduces many microscopic wear particles into the surrounding tissues. The reaction to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the prosthesis is anchored. Eventually, the prosthesis becomes painfully loose and must be replaced.
Improving the wear resistance of the UHMWPE socket and, thereby, reducing the rate of production of wear debris would extend the useful life of artificial joints and permit them to be used successfully in younger patients. Consequently, numerous modifications in physical properties of UHMWPE have been proposed to improve its wear resistance.
UHMWPE components are known to undergo a spontaneous, post-fabrication increase in crystallinity and changes in other physical properties. {See e.g., Rimnac, C. M., et al., J. Bone & Joint Surgery, 76-A(7):1052-1056 (1994)}. These changes occur even in stored (non-implanted) cups after sterilization with gamma radiation, which initiates an ongoing process of chain scission, crosslinking, and oxidation or peroxidation involving the free radicals formed by the irradiation. These degradative changes may be accelerated by oxidative attack from the joint fluid and cyclic stresses applied during use.
In an attempt to improve wear resistance, DePuy-DuPont Orthopaedics fabricated acetabular cups from conventionally extruded bar stock that previously had been subjected to heating and hydrostatic pressure that reduced fusion defects and increased the crystallinity, density, stiffness, hardness, yield strength, and increased the resistance to creep, oxidation and fatigue. Alternatively, silane cross-linked UHMWPE (XLP) has also been used to make acetabular cups for total hip replacements in goats. In this case, the number of in vivo debris particles appeared to be greater for XLP than conventional UHMWPE cup implants {Ferris, B. D., J. Exp. Path., 71:367-373 (1990)}.
Other modifications of UHMWPE have included: (a) reinforcement with carbon fibers; and (b) post-processing treatments such as solid phase compression molding. Indeed, carbon fiber reinforced polyethylene and a heat-pressed polyethylene have shown relatively poor wear resistance when used as the tibial components of total knee prosthesis. {See e.g., Rimnac, C. M., et al., Trans. Orthopaedic Research Society, 17:330 (1992)}.
Recently, several companies have modified the method of radiation sterilization to improve the wear resistance of UHMWPE components. This has typically involved packaging the polyethylene cups either in an inert gas (e.g., Howmedica, Inc.), in a partial vacuum (e.g., Johnson & Johnson, Inc.) or with an oxygen scavenger (e.g., Sulzer Orthopaedics, Inc.).
The present invention comprises two aspects:
The first aspect of the invention presents a method for increasing the wear resistance of a polymer by crosslinking the polymer, followed by thermally treating the crosslinked polymer. Non-limiting examples of the thermal treatments are remelting or annealing. Preferably, the polymer is crosslinked by gamma irradiation in the solid state prior to being modified to a desired final form or shape of the final product. In the preferred embodiment, the surface layer of the crosslinked and thermally treated polymer, which is the most oxidized and least crosslinked part of the polymer, is removed, e.g., in the process of machining the final product out of the irradiated bar and thermally treated bar or block. The radiation dose is also preferably adjusted so that the optimal dose occurs within the solid polymer bar or block at the level of the bearing surface of the final product. Also presented are the polymers made from this method; methods for making products (e.g., in vivo implants) from these polymers; and the products (e.g., in vivo implants) made from these polymers.
The second aspect of the invention provides a systematic method for determining an optimal balance among wear resistance and other physical and/or chemical properties that are deemed important to the long-term performance of an implant in vivo, and applying this optimal balance to determine the appropriate crosslinking and thermal treatment conditions for processing a polymer. A flowchart is provided as a non-limiting illustration of the method for determining the optimal balance. Also provided are methods for treating polymers which apply the above appropriate crosslinking and thermal treatment conditions; the polymers produced by these methods; methods for making products (e.g., in vivo implants) from these polymers; and the products (e.g., in vivo implants) made from these polymers.
Abbreviations used in this application are as follows:
The present invention contains two aspects. The first aspect of the invention provides methods for improving the wear resistance of a polymer by crosslinking (preferably the bearing surface of the polymer) and then thermally treating the polymer, and the resulting novel polymer. Preferably, the most oxidized surface of the polymer is also removed. Also presented are the methods for using the polymeric compositions for making products and the resulting products, e.g., in vivo implants. Specific examples of this method are presented in the section: “I. First Aspect of the Invention: Polymeric Compositions with Increased Wear Resistance” and “I (A) Further Examples of the First Aspect of the Invention”, below.
The method of the invention utilizes irradiation for crosslinking a polymer followed by thermal treatment to decrease the free radicals to produce a preformed polymeric composition. The term “preformed polymeric composition” means that the polymeric composition is not in a final desired shape or form (i.e., not a final product). For example, where the final product of the preformed polymeric composition is an acetabular cup, irradiation and thermal treatment of the polymer could be performed at pre-acetabular cup shape, such as when the preformed polymeric composition is in the form of a solid bar or block.
A second aspect of the invention provides a systematic method (an example of which is illustrated in the flowchart, below) for determining the optimal parameters for the above mentioned crosslinking and thermal treatment. This second aspect provides a method for determining the maximum possible improvement in wear resistance, consistent with keeping the other physical and/or chemical propert(ies) within the user's desired limits, with the least amount of trial and error testing. Once the optimal parameters (i.e., crosslinking conditions such as radiation dose when radiation is used to crosslink the polymer, and thermal treatment parameters) are determined by this method, the polymer will then be processed according to the optimal parameters. Thus, this protocol renders the development of a preformed polymeric composition with particular chemical/mechanical characteristics routine without resort to undue experimentation. Also presented are the methods for using the preformed polymeric composition for making products, and the products, e.g., in vivo implants.
In the present invention, the wear resistance of the polymer is improved by crosslinking. The crosslinking can be achieved by various methods known in the art, for example, by irradiation from a gamma radiation source or from an electron beam, or by photocrosslinking. The preferred method for crosslinking the polymer is by gamma irradiation. The polymer is preferably crosslinked in the form of an extruded bar or molded block.
In the preferred method, the crosslinked polymer is subjected to thermal treatment such as by remelting (i.e., heated above the melting temperature of the crosslinked polymer) or annealing (i.e., heated at below the melting temperature of the crosslinked polymer) to produce the preformed polymeric composition.
In the preferred embodiment of both the first and second aspects of the invention, the outer layer of the resulting preformed polymeric composition, which is generally the most oxidized and least crosslinked and, thus, least wear resistant, is removed. For example, the bearing surface of the preformed polymeric composition may be fashioned from inside, e.g., by machining away the surface of the irradiated and thermally treated composition before or during fashioning into the final product, e.g., into an implant. Bearing surfaces are surfaces which are in moving contact with another, e.g., in a sliding, pivoting, or rotating relationship to one another.
Choices of Polymers
The polymers are generally polyester, poly(methylmethacrylate), nylon, polycarbonates, and polyhydrocarbons such as polyethylene, and polypropylene. High molecular weight (HMW) and ultra-high molecular weight (UHMW) polymers are preferred, such as HMW polyethylene (HMWPE), UHMW polyethylene (UHMWPE), and UHMW polypropylene. HMW polymers have molecular weights ranging from about 105 grams per mole to just below 106. UHMW polymers have molecular weights equal to or higher than 106 grams per mole, preferably from 106 to about 107. The polymers are generally between about 400,000 grams per mole to about 10,000,000 and are preferably polyolefinic materials.
For implants, the preferred polymers are those that are wear resistant and have exceptional chemical resistance. UHMWPE is the most preferred polymer as it is known for these properties and is currently widely used to make acetabular cups for total hip prostheses and components of other joint replacements. Examples of UHMWPE are those having molecular weight ranging from about 1 to 8×106 grams per mole, examples of which are: GUR 4150 or 4050 (Hoechst-Celanese Corporation, League City, Tex.) with a weight average molecular weight of 5 to 6×106 grams per mole; GUR 4130 with a weight average molecular weight of 3 to 4×106; GUR 4120 or 4020 with a weight average molecular weight of 3 to 4×106; RCH 1000 (Hoechst-Celanese Corp.) with a weight average of molecular weight of 4×106 and HiFax 1900 of 2 to 4×106 (HiMont, Elkton, Md.). Historically, companies which make implants have used polyethylenes such as HIFAX 1900, GUR 4020, GUR 4120 and GUR 4150 for making acetabular cups.
Sterilization Methods
All polymeric products must be sterilized by a suitable method prior to implanting in the human body. For the formed crosslinked and thermally treated polymeric compositions (i.e., the final products) of the present invention, it is preferable that the products be sterilized by a non-radiation based method, such as ethylene oxide or gas plasma, in order not to induce additional crosslinking and/or oxidation of the previously treated preformed polymeric composition. Compared to radiation sterilization, a non-radiation sterilization method has a minor effect on the other important physical characteristics of the product.
Nevertheless, the method can be used in conjunction with radiation sterilization. If the final products are to be sterilized by an additional dose of radiation, it is preferable to take into account the effect of this additional radiation dose on the wear resistance and other properties of the polymer, in determining the optimum radiation dose used in the initial crosslinking. Furthermore, it is preferable that the radiation sterilization be done while the final product (e.g., in vivo implant) is packed in a suitable low-oxygen atmosphere (e.g., in partial vacuum, in an inert gas such as nitrogen, or with an oxygen scavenger included) in order to minimize oxidation of the surface layer of the final product during and after sterilization by irradiation.
The dose ranges in this application do not take into account radiation sterilization. If radiation sterilization is used, then the dose ranges may have to be adjusted. Such adjustment can be easily performed using the teachings herein. For example, if after comparing the dose-response curves for wear with those for other important physical or chemical properties, it is determined that the optimal total radiation dose is 8 Mrad, and it is intended to sterilize the polymer with 2.5 Mrad gamma radiation (the minimum industrial standard sterilization dose), then the initial radiation dose (before sterilization) should be 5.5 Mrad, such that the total dose (initial plus sterilization doses) will be 8 Mrad. These calculations are approximate, since the total crosslinking achieved will not be exactly equivalent to a single 8 Mrad dose.
Nevertheless, the applicants have discovered that a high level of crosslinking in the surface layer of a polymer markedly reduces the degradative effects of surface oxidation, i.e., that would otherwise occur if a non-pre-crosslinked polymer were irradiated in the presence of oxygen (for example, see
Methods for Characterizing the Polymers
The degree of crystallinity can be determined using methods known in the art, e.g. by differential scanning calorimetry (DSC), which is generally used to assess the crystallinity and melting behavior of a polymer. Wang, X. & Salovey, R., J. App. Polymer Sci., 34:593-599 (1987).
Wide-angle X-ray scattering from the resulting polymer can also be used to further confirm the degree of crystallinity of the polymer, e.g. as described in Spruiell, J. E., & Clark, E. S., in “Methods of Experimental-Physics”, L. Marton & C. Marton, Eds., Vol. 16, Part B, Academic Press, New York (1980). Other methods for determining the degree of crystallinity of the resulting polymer may include Fourier Transform Infared Spectroscopy {Painter, P. C. et al., “The Theory Of Vibrational Spectroscopy And Its Application To Polymeric Materials”, John Wiley and Sons, New York, U.S.A. (1982)} and density measurement (ASTM D1505-68). Measurements of the gel content and swelling are generally used to characterize crosslink distributions in polymers, the procedure is described in Ding, Z. Y., et al., J. Polymer Sci., Polymer Chem., 29:1035-38 (1990). FTIR can also be used to assess the depth profiles of oxidation as well as other chemical changes such as unsaturation {Nagy, E. V., & Li, S., “A Fourier transform infrared technique for the evaluation of polyethylene orthopaedic bearing materials”, Trans. Soc. for Biomaterials, 13:109 (1990); Shinde, A. & Salovey, R., J. Polymer Sci., Polym. Phys. Ed., 23:1681-1689 (1985)}.
Use of Crosslinked Polymers for Implants
Another aspect of the invention presents a process for making implants using the preformed polymeric composition of the present invention. The preformed polymeric composition may be shaped, e.g., machined, into the appropriate implants using methods known in the art. Preferably, the shaping process, such as machining, removes the oxidized surface of the composition.
Preformed Polymeric Compositions
The preformed polymeric compositions of the present invention can be used in any situation where a polymer, especially UHMWPE, is called for, but especially in situations where high wear resistance is desired. More particularly, these preformed polymeric compositions are useful for making implants.
Implants Made of Crosslinked Polymers
An important aspect of this invention presents implants that are made with the above preformed polymeric compositions or according to the methods presented herein. In particular, the implants are produced from preformed polymeric composition are UHMW polymers crosslinked by gamma radiation followed by remelting or annealing, removing the oxidized surface layer and then fabricating into a final shape. The preformed polymeric composition of the present invention can be used to make implants for various parts of the body, such as components of a joint in the body. For example, in the hip joints, the preformed polymeric composition can be used to make the acetabular cup, or the insert or liner of the cup, or trunnion bearings (e.g. between the modular head and the stem). In the knee joint, the preformed polymeric composition can be used to make the tibial plateau (femoro-tibial articulation), the patellar button (patello-femoral articulation), and trunnion or other bearing components, depending on the design of the artificial knee joint. In the ankle joint, the preformed polymeric composition can be used to make the talar surface (tibio-talar articulation) and other bearing components. In the elbow joint, the preformed polymeric composition can be used to make the radio-numeral joint, ulno-humeral joint, and other bearing components. In the shoulder joint, the preformed polymeric composition can be used to make the glenoro-humeral articulation, and other bearing components. In the spine, the preformed polymeric composition can be so used to make intervertebral disk replacement and facet joint replacement. The preformed polymeric composition can also be made into temporo-mandibular joint (jaw) and finger joints. The above are by way of example, and are not meant to be limiting.
The following discusses the first and second aspects of the invention in more detail.
I. First Aspect of the Invention: Polymeric Compositions with Increased Wear Resistance
The first aspect of the invention provides preformed polymeric compositions which are wear resistant and useful for making in vivo implants. In this aspect, for polymers in general, and more preferably UHMW and HMW polymers, and most preferably UHMWPE and HMWPE, the irradiation dose is preferably from about 1 to about 100 Mrad, and more preferably, from about 5 to about 25 Mrad, and most preferably from about 5 to about 10 Mrad. This most preferable range is based on achieving what the inventors have determined to be a reasonable balance between improved wear resistance and minimal degradation of other important physical properties.
In vivo implants of the present invention, i.e., irradiated within the above dose ranges are expected to function in vivo without mechanical failure. The UHMWPE acetabular cups used by Oonishi et al. [in Radiat. Phys. Chem., 39: 495-504 (1992)] were irradiated to 100 Mrad and functioned in vivo without reported mechanical failure after as long as 26 years of clinical use. Furthermore, it is surprising that, as shown in the EXAMPLES, acetabular cups from the preformed polymeric composition prepared according to the present invention, but irradiated to much less than 100 Mrad, exhibited much higher wear resistance than reported by Oonishi et al.
On the other hand, if a user is primarily concerned with reducing wear, and other physical properties are of secondary concern, then a higher dose than the above stipulated most preferable range (e.g., 5 to 10 Mrad) may be appropriate, or vice versa (as illustrated in the detailed examples in the following section). The optimum radiation dose is preferably based on the dose received at the level of the bearing surface in the final product. Gamma radiation is preferred.
The irradiated polymer is then preferably remelted at or above melting temperature of the irradiated polymer, e.g., in air. As used herein, the melting temperature of the crosslinked or irradiated polymer is identified from the peak of the melting endotherm as measured by DSC. Preferably, the remelting temperature is from about the melting temperature of the irradiated polymer to about 100° C. to about 160° C. above the melting temperature of the irradiated polymer; more preferably from about 40° C. to about 80° C. above the melting temperature of the irradiated polymer; and most preferably from about 1° C. to about 60° C. above the melting temperature of the irradiated polymer. For example, in the case of UHMWPE, the remelting temperature is preferably from about 136° C. to about 300° C., more preferably from about 136° C. to about 250° C., and most preferably from about 136° C. to about 200° C. Specific conditions for remelting are described in EXAMPLES 1 and 2, below.
Generally, in practice, the remelting temperature is inversely proportional to the remelting period. The polymer is preferably remelted over a period from about 1 hour to about 2 days, more preferably from about 1 hour to about 1 day, and most preferably from about 2 hours to about 12 hours.
Since, depending on the time and temperature applied, annealing can produce less of an effect than remelting on physical properties such as crystallinity, yield strength and ultimate strength, annealing may be used in place of remelting as a means for reducing the free radicals remaining in the polymer after irradiation crosslinking, in order to maintain these physical properties within limits required by the user. Thermal treatment, such as remelting or annealing, removes free radicals and thereby improves long term wear resistance of the polymer. On the other hand, annealing is slower and thus takes longer than remelting, making it likely to be more expensive in industrial applications.
The annealing temperature is preferably from about room temperature to below the melting temperature of the irradiated polymer; more preferably from about 90° C. to about 1° C. below the melting temperature of the irradiated polymer; and most preferably from about 60° C. to about 1° C. below the melting temperature of the irradiated polymer. For example, UHMWPE may be annealed at a temperature from about 25° C. to about 135° C., preferably from about 50° C. to about 135° C., and more preferably from about 80° C. to about 135° C. The annealing period is preferably from about 2 hours to about 7 days, and more preferably from about 7 hours to about 5 days, and most preferably from about 10 hours to about 2 days.
Instead of using the above range of radiation dose as a criterion, the appropriate amount of crosslinking may be determined based on the degree of swelling, gel content, or molecular weight between crosslinks after thermal treatment. This alternative is based on the applicant's findings (detailed below) that acetabular cups made from UHMWPE falling within a preferred range of these physical parameters have reduced or non-detectable wear. The ranges of these physical parameters include one or more of the following: a degree of swelling of between about 1.7 to about 5.3; molecular weight between crosslinks of between about 400 to about 8400 g/mol; and a gel content of between about 95% to about 99%. A preferred polymer or final product has one or more, and preferably all, of the above characteristics. These parameters can also be used as starting points in the second aspect of the invention (as illustrated by the flowchart, discussed below) for determining the desired radiation dose to balance the improvement in wear resistance with other desired physical or chemical properties, such as polymer strength or stiffness.
After crosslinking and thermal treatment, preferably, the most oxidized surface of the preformed polymeric composition is removed. The depth profiles of oxidation of the preformed polymeric composition can be determined by methods known in the art, such as FTIR, described above and in EXAMPLES 3 and 6. In general, to remove the most oxidized surface, preferably a minimum of about 0.5 mm to 1.0 mm of the surface of preformed polymeric composition which is exposed to air is removed, e.g. by machining, before or while fashioning the preformed polymeric composition into the final product.
I. (A) Further Examples of the First Aspect of the Invention
As noted above, the most preferable range of dose for crosslinking radiation (i.e., from 5 to 10 Mrad) was based on achieving what the inventors have determined to be a reasonable balance between improved wear resistance and minimal degradation of other important physical properties. The following examples illustrate applications of the present invention with alternative criteria for the optimal dose. These examples use in vivo implants as non-limiting examples of the products, and UHMWPE or HMWPE bar or block as a non-limiting example of a starting material.
In the first example, the user desires to achieve a minimum wear rate of the in vivo implant made from the UHMWPE and HMWPE, and the other physical or chemical properties are important but of lesser concern. In such a case, the user may choose to irradiate the UHMWPE and HMWPE bar or block between about 15 Mrad to about 20 Mrad (as shown by so
In a second example, the user may wish to produce an UHMWPE which is as wear resistant as possible while meeting the tensile strength at break (ultimate), tensile strength at yield, and elongation at break criteria of the standard specified by the American Society for Testing and Materials F-648 standard (hereinafter referred to as “ASTM F648”) for UHMWPE for in vivo use. The information about this standard can be found in a current issue of the Annual Book of ASTM Standards, Medical Devices and Services, “Standard Specification for Ultra-High-Molecular-Weight Polyethylene Powder and Fabricated Form for Surgical Implants”, American Society for Testing and Materials. The method of the second aspect of the present invention (as illustrated by the flowchart) may be used to adjust the crosslinking and thermal treatment parameters to meet any current ASTM F648 criteria.
For example, to meet the 1996 ASTM F648 (F648-96) criteria for Type 1 or 2 UHMWPE, the UHMWPE must have: a tensile strength at break (ultimate) of at least 35 MPa (for Type 1) and 27 MPa (for Type 2) at 23° C. and 5.08 cm/min; a tensile strength at yield of at least 21 MPa (Type 1) and 19 MPa (for Type 2) at 23° C., and 5.08 cm/min; and elongation at break of at least 300% at 5.08 cm/min. The test conditions are described in ASTM D638, Type IV (Annual Book of ASTM Standards, American Society for Testing and Materials). Alternatively, to meet the 1996 ASTM F648 criteria for Type 3 UHMWPE, the UHMWPE must have: a tensile strength at break (ultimate) of at least 27 MPa at 23° C. and 5.08 cm/min; a tensile strength at yield of at least 19 MPa at 23° C., and 5.08 cm/min; and elongation at break of at least 250% at 5.08 cm/min.
The plots of mechanical properties vs irradiation dose for GUR 4150 (which is representative of Type 2 UHMWPE) (
II. Second Aspect of the Invention: Method for Optimizing Wear Resistance and Desirable Physical and/or Chemical Characteristics of a Polymeric Composition
The second aspect of the invention uses the findings in this patent application (including those presented in the “EXAMPLES” section, below) to construct a method which allows one skilled in the art, to systematically identify the conditions necessary to routinely produce a polymer with an optimal balance of wear resistance and physical and/or chemical properties, with minimal additional testing and minimal trial and error. In one embodiment of this aspect of the invention, the optimizing method can be schematically illustrated in a flowchart. Once the optimal conditions have been determined by this method, the polymer can then be subjected to these conditions for processing.
The present invention is based in part, on the discovery that wear rate decreases with increasing radiation dose, and there is a maximum dose above which there is little or no additional improvement in wear, but higher doses might degrade other important physical and/or chemical properties of the polymer, such as yield or ultimate strength, elongation to failure, impact strength or fatigue resistance, as well as increasing the susceptibility to oxidation. Oxidation, in turn, is known to adversely affect one or more of these physical properties, and was shown to occur in the examples below for UHMWPE crosslinked at a dose averaging about 28 Mrad if there had been no thermal treatment. Consequently, a polymer irradiated at a high radiation dose may exhibit improved wear resistance, but its other physical or chemical properties may fall outside of desirable or allowable limits, such as those specified by ASTM F648 for UHMWPE for in vivo use.
The method is also based in part on the discovery that, while other important physical properties (such as crystallinity or elongation to break) may be markedly affected by the amount of thermal treatment (e.g., remelting or annealing) applied to the polymer after irradiation crosslinking, the wear resistance is not markedly affected. This latter discovery permits reducing the amount of additional testing required by the user in order to identify the crosslinking dose which will provide the user's desired balance among wear resistance and other physical properties. This method is useful, e.g., in the case where performed polymeric composition made of UHMWPE is used for making in vivo implants, such as acetabular cups.
II (a) Summary of the Steps of the Optimization Method
Thus, the second aspect of the present invention provides a systematic method for optimizing the balance among wear resistance and other desired physical and/or chemical characteristics of a polymer. The steps in this method are summarized in the non-limiting example of the flowchart of
Step 1: The process typically begins with the polymer in solid form, such as an extruded bar or block.
Step 2: The bar is irradiated over a range of doses up to the maximum that is likely to produce a material with the desired wear resistance and physical and/or chemical properties. This irradiation may be done, for example, in the case of gamma radiation by means of a cobalt 60 gamma radiation facility as is presently used for industrial-scale sterilization of implants.
Step 3: The irradiated bars are then remelted. Applicants found that remelting of an irradiated polymer would substantially reduce the free radicals produced during irradiation, thus minimizing long-term oxidation and chain scission. By improving the polymeric composition's resistance to long-term oxidation, remelting also improves the polymeric composition's long-term resistance to wear. For further discussion of the subject, see EXAMPLES 2, 3, and 4, below
Although the bar may be contained in a low-oxygen atmosphere during the remelting, this may not be essential since, even if the bar is remelted in ambient air, the resultant oxidation may affect only the surface layer of the polymer (e.g. in the following EXAMPLE section,
Step 4A The radiation dose is correlated with the wear resistance of the products made from the irradiated remelted polymeric composition, as determined in a wear test that adequately simulates the wear conditions of the products. For example, if the polymeric composition will be made into an implant, then the wear test should preferably adequately simulate the wear conditions of such implants in vivo. The correlation may be arrived at by plotting a dose-response curve for irradiation dose vs. wear.
Step 4B: Similarly, the radiation dose is correlated with each of the physical and/or chemical properties that may be markedly affected by the radiation dose and that might, in turn, substantially affect the performance of the implant in vivo, both for non-remelted and remelted polymer. Again, the correlation may be arrived at by plotting a dose-response curve for irradiation dose vs. each of these physical and/or chemical properties.
The user does not have to do dose vs. properties for each property that might be affected, but only those properties that are considered important for the proper functioning of the implant in vivo. Which of these properties are important for the intended application, and the limiting values of these properties, may vary for different polymeric compositions and for different types of applications (e.g., hip prostheses compared to knee prostheses) and must, therefore, be established by the user before applying the flowchart.
Step 5 is the first attempt at optimization. The user may first decide on the desired amount of improvement in the wear resistance, i.e., the maximum wear rate that is permissible for the user's application. The dose-response curve for wear (Step 4A) then shows the minimum radiation dose necessary to provide this amount of improvement in wear resistance.
Similarly, the dose response curves for the other physical or chemical properties deemed critical or important (Step 4B) provide the values of these properties corresponding to the specific radiation dose identified in Step 4A as being necessary to provide the desired improvement in wear resistance. If each of these other physical or chemical properties are within allowable limits for the crosslinked and remelted polymer, then an optimal method has been identified (Step 6). In other words, the implant can be made by irradiating the solid polymer bar, remelting the bar and machining out the implant; the entire process being conducted such that the resulting implant has received the optimal dose at its bearing surface.
Alternatively, the user may first decide on critical values for one or more properties, such as ultimate tensile strength, fatigue strength, etc., and then check the corresponding dose response curves for the remelted polymer for the maximum allowable dose, and then check the wear vs. dose curve to determine whether this dose gives sufficient improvement in wear (i.e., the user does not necessarily have to begin by choosing the desired amount of improvement in wear).
However, if not enough improvement in wear will be obtained while keeping these other chemical and physical properties within allowable limits, or conversely, if the dose required for the desired wear improvement causes one or more of these properties to be out of allowable limits, then the user can use a lower radiation dose (i.e., accept a higher wear rate) if he wishes to remelt the materials or, alternatively, annealing may be substituted for remelting (Step 7). For a crosslinked material, annealing is less efficient than remelting in removing free radicals, but may cause less of a reduction in other important physical properties.
Whether annealing is a practical option will be apparent from the dose-response curves for the non-remelted and remelted polymers. That is, if the desired value of the property in question falls between the two curves (see for example,
It is not necessary to generate additional wear dose-response curves for each of the many possible combinations of annealing time and temperature. It is expected that the radiation dose necessary to produce the desired reduction in wear that is determined from the wear dose-response curve for remelted polymer in Step 4A, will also apply to an annealed polymer produced in Step 7.
Step 7: Anneal samples of a bar or block which have been irradiated to that dose that was identified in Step 4A as being necessary to provide the required improvement in wear resistance, at various time/temperature combinations, to produce a polymer with the critical properties between those for non-remelted and remelted materials.
Step 8: The physical or chemical propert(ies) of interest of the irradiated and annealed samples of the polymer are correlated with annealing times and temperatures.
Step 9: Using ultimate tensile strength as an example of the physical characteristic of interest, depending on the resultant curve for annealing time and/or temperature vs. ultimate strength, the radiation dose required to achieve the desired wear resistance identified in Step 4A (above) should produce a polymer with an ultimate strength within allowable limits.
Similar consideration should be given to each of the other important physical and/or chemical properties by generation of individual curves of these properties versus annealing time and/or temperature. If each of these properties is within allowable limits at a particular annealing time and temperature combination, then a suitable method has been identified (Step 10).
If an annealing process cannot be identified that maintains the properties within allowable limits, then the user may choose to accept a lower radiation dose (Step 11), i.e., to accept less of an improvement in wear resistance. However, if a lower radiation dose (and, therefore, a greater wear rate) is acceptable, then the corresponding physical and chemical properties should again be checked for the remelted polymer (using the correlation arrived earlier in Step 4B), since these may be within limits at the lower radiation dose.
If the properties are within limits for the remelted polymer at the lower radiation dose, then remelting may be used instead of annealing to produce a polymer with the desired improvement in wear resistance (Step 6). If not, then the user should proceed with annealing as before (Steps 7 through 10 or 11) but at this lower radiation dose.
The user may wish to progressively reduce the required amount of radiation crosslinking (i.e., to accept still higher wear rate) until a dose is identified for which all of the other properties deemed essential are within the user's required limits. The resultant dose represents the maximum improvement in wear resistance obtainable within the user's criteria.
II (b) Example Applications of the Flowchart
As starting points for the flowchart, the ranges for radiation doses, remelting and annealing temperatures and times described in section “I. First Aspect of the Invention: Polymeric Compositions with Increased Wear Resistance” and “I (A) Further Examples of the First Aspect of the Invention”, above, can be used, with regard to polymers in general, UHMW and HMW polymers in particular, and HMWPE and UHMWPE especially.
For ease of discussion, the following examples illustrate the application of the flowchart using UHMWPE (which also behaves similar to HMWPE) as an example of a polymer and an acetabular cup as an example of an implant. GUR 4150 is representative of such a class of UHMWPE. Similarly, the description uses gamma radiation as an example for crosslinking the polymer. These examples are meant to illustrate and not meant to limit the invention.
The method described by the flowchart is applicable to other polymers, implants or other products made from such polymers, and crosslinking methods (examples of which are described elsewhere in this application), and methods for making an implant or product out of the preformed polymeric composition.
From the data provided by the EXAMPLES (following sections) a number of generalities were discovered that allowed simplification of the use of the flowchart, i.e., to minimize the amount of additional testing that would be required of a user wishing to apply the method to other polymers, or to the GUR4150 of the EXAMPLES but with various optimization criteria.
To establish the critical curve for the reduced in vivo wear (Step 4A), the UHMWPE bar or block is preferably irradiated in Step 2 and remelted in Step 3, in a manner and to a dose and temperature and time as described for UHMWPE in the section, “I. First Aspect of the Invention: Polymeric Compositions with Increased Wear Resistance” and “I (A) Further Examples of the First Aspect of the Invention”, above.
In step 4A, acetabular cups are machined out of the irradiated bar and wear tested under conditions suitably representative of the intended in vivo application (e.g., by the method described in the EXAMPLES section below) to establish a wear vs. radiation dose response curve for the specific polymer. EXAMPLE 5 and
Applicants discovered that it is not necessary to generate additional wear dose-response curves for each of the many possible combinations of annealing time and temperature. This follows from the results of EXAMPLE 2. Since annealing is done at a lower temperature than remelting and, therefore, has a less marked effect on physical properties in general, it can be expected that annealing will have even less of an effect on the wear resistance than remelting.
Another important aspect of the invention is the discovery that wear resistance of GUR 4150 was not markedly affected by remelting and, therefore, it is also not likely to be markedly affected by annealing time and temperature. Therefore, it is expected that the radiation dose necessary to produce the desired reduction in wear that is determined from the wear dose-response curve for remelted polymer in Step 4A, will also apply to an annealed polymer produced in Step 7. Therefore, while the user needs to do his own tests to establish tensile strength vs dose etc., he can rely on the wear vs dose curve developed for the remelted material, rather than running an additional set of wear curves for each annealing condition. This represents a considerable saving in experimental costs, since the tensile strength tests typically may be completed in a few days (using common tensile test apparatus), but the tests of wear vs dose require months to complete (and require highly specialized equipment and techniques available on only a handful of laboratories in the world).
Furthermore, if the user is working with GUR 4150, he can use the dose vs wear curve of
For other polymers, comparable wear tests at each end of the range of interest for radiation dose could be applied to verify whether remelting or annealing also does not markedly affect their wear resistance. Nevertheless, GUR 4150 is representative of UHMWPEs, especially those useful for implants, in its physical and chemical properties, and applicants have observed that other UHMWPEs, of different molecular weights and with or without calcium stearate, such as GUR 1020 (calcium stearate free, lower molecular weight grade) behaved similarly to GUR 4150 in their wear resistance after irradiation sterilization in air. McKellop, H. et al., Trans. Society for Biomaterials, Vol. 20, pg. 43 (1997).
Further, it has been observed that, although the starting physical properties of HMWPE are different from those of UHMWPE, these differences will be substantially reduced after sufficient crosslinking. For example, they are almost equal after electron beam irradiation treatment to 300 kGy (30 Mrad), for properties like gel content, swelling and strength. Streicher, R. M., Beta-Gamma 1/89: 34-43, at p. 42, right col., fourth full paragraph. Even the wear properties were the same, after the differences in the molecular arrangement between HMWPE and UHMWPE were offset by the irradiation procedure. Thus, it is predicted that the findings based on GUR 4150 and the above discussion would be applicable to polymers in general, and to UHMW and HMV polymers, in particular, and especially HMWPE and UHMPE. Thus, the radiation, remelting and annealing ranges found for GUR 4150 can be applied to polymers in general, and more preferably to HMW and UHMW polymers, and most preferably to HMWPE and UHMWPE; and these ranges can be used at the very least, as starting points in the flowchart for determining the specific ranges for other polymers, and the data in the “EXAMPLES” section below will facilitate the user in arriving at the proper conditions for GUR 4150, ASTM F648 Type 2 UHMWPE, and UHMWPE and HMWPE in general.
The following examples illustrate the use of these generalities in conjunction with the flowchart. In the first example, if the user is working with GUR 4150, or an UHMWPE that satisfies ASTM F648 Type 2 criteria in general, then, based on
In a second example, a user requires a lower limit on tensile strength at break at 40 MPa, and wishes to produce a material with wear no more than 1 mm3 per million cycles. The wear vs dose curve (
Having described the invention, the following examples are presented to illustrate and support the invention, and are not to be construed as limiting the scope of the invention.
The nominal dose of radiation applied to implants at a commercial radiation facility typically varies within a range. Therefore, in the following examples, the average gamma radiation doses are given, such as average gamma radiation doses of 3.3, 26.5, and 28 Mrad. The average of 3.3 Mrad was arrived at by averaging the minimum and maximum doses, e.g., a minimum of 3.28 and a maximum of 3.45 Mrad. Similarly, for example, the average of 26.5 was based on averaging a minimum of 25.14 and a maximum of 27.70 Mrad; and the average of 28 was based on averaging a minimum of 26.01 and a maximum of 30.30 Mrad.
Experimental Details
Commercial-grade UHMWPE extruded bars (GUR 4150, Poly Hi Solidur), with a weight average molecular weight of 5-6×106, were used as received. The 8 mm thick specimens were cut from the bars and irradiated with gamma-rays at room temperature either in ambient air or in a vacuum chamber at SteriGenics International (Tustin, Calif.) to average doses ranging from 3.3 to 250 Mrad. Radiation was delivered at a dose rate of 0.2 Mrad/hr. For 250 Mrad, the dose rate was 4 Mrad/hr. Cobalt-60 was used as a source of gamma radiation. A subset of the 8 mm thick specimens that had been irradiated in vacuum was remelted in a vacuum oven by heating from room temperature to 145° C. slowly (at about 0.3° C./min.) and maintaining at 145° C. for one hour. After remelting, the specimens were slowly cooled to room temperature.
The physical properties of the disk specimens before and after irradiation and remelting were characterized by DSC, gel content analysis and FTIR.
Gel Content Analysis
The gel content of each material was analyzed as a function of depth from the surface. 100 μm thick sections (about 50 mg) were microtomed across the specimen. Extraction of the sol-fraction was performed by boiling in p-xylene for 24 hours, with 0.5 wt % of antioxidant (2,6-di-t-butyl-4-methyl phenol) added to prevent oxidation. For highly oxidized sections from the surface layer, which tended to break up during boiling, the specimens were wrapped in PTFE membrane filter (0.5 μm pore size) to avoid loss of gel. After extraction, the specimens were de-swollen in acetone and dried at 60° C. in a vacuum oven to constant weight. The gel fraction was determined from the ratio of the weight of the dried-extracted material to that of the dry non-extracted material.
Differential Scanning Calorimetry (DSC)
For DSC measurements, samples were cored and microtomed into 200 μm thick sections across the depth. Specimens (˜4 mg) were heated from 50° C. at 10° C./min in a differential scanning calorimeter (Perkin-Elmer DSC-4) to 170° C. The melting temperature was identified from the peak of the melting endotherm. Indium was used for calibration of the temperature and heat of fusion. The heat of fusion was determined by comparing the area under the melting endotherm to the area of fusion of an indium sample having a known heat of fusion of 28.4 J/g, and divided by 292 J/g, the heat of fusion of an ideal polyethylene crystal, to get the degree of crystallinity.
Results and Discussion
As shown in
In a second phase of this example, the 8 mm thick disks that had been irradiated in vacuum were remelted by heating to 145° C. for one hour, and slowly cooled. This reduced the peak melting temperature, the degree of crystallinity and the crystal size. For example (
In addition, during remelting, residual free radicals that were formed by the irradiation apparently recombined and increased the total crosslinking (as evident from the increased gel content,
The lower gel content (crosslinking) near the surface (
The crystallinity and gel content of the irradiated 8 mm thick disks, with and without remelting, are compared in
Experimental Details
Six extruded bars of UHMWPE (GUR 4150), each 3 inches in diameter, were exposed to 3.3 or 28 Mrad of gamma radiation at a dose rate of 0.2 Mrad per hour in ambient air (SteriGenics, Inc., Tustin, Calif.). Two bars for each radiation dose were then remelted by heating in an oven in ambient atmosphere from room temperature to 150° C. at about 0.3° C. per minute and holding at 150° C. for five hours, and then slow-cooling to room temperature. The crystallinity and gel content of these four materials were measured across the cross section of extra samples of each bar using differential scanning calorimetry (DSC) and gel content analysis. The results are summarized in Tables 1 and 2.
Four sets of acetabular cups were machined from bars of each of the four materials at a commercial machining shop (Bradford and Meneghini Manufacturing Co., Santa Fe Springs, Calif.). Each cup had a 2 inch outer diameter (O.D.) and 1.26 inch inner diameter (I.D.), and 1 inch outer radius and 0.633 inch inner radius (
Since acetabular cups used in patients must first be sterilized by some acceptable means, the test cups in this study were sterilized prior to wear testing using ethylene oxide at the appropriate dose for clinical implants. Ethylene oxide was chosen instead of additional gamma irradiation (e.g., 2.5-4.0 Mrad) in order to confine the results to the effects of the original 3.3 or 28 Mrad doses used to crosslink the materials.
Prior to wear testing, the cups were pre-soaked in distilled water for three weeks to minimize additional fluid absorption during the wear test, thereby making the weight loss method for wear measurement more accurate. For the wear test, the cups were enclosed in polyurethane molds and pressed into stainless steel holders (
During the test, the bearing surfaces were kept immersed in bovine blood serum to simulate lubrication in the human body. Sodium azide at 0.2% was added to the serum to retard bacterial degradation, and 20 mM ethylene-diaminetetraacetic acid (EDTA) was added to prevent precipitation of calcium phosphate onto the surface of the balls (McKellop, H. and Lu, B., “Friction and Wear of Polyethylene-metal and Polyethylene-ceramic Hip Prostheses on a Joint Simulator, Transactions of the Fourth World Biomaterials Congress, Berlin, April 1992, p. 118). A polyethylene skirt covered each test chamber to minimize airborne contaminants.
At intervals of 250,000 cycles, the cups were removed from the machine, rinsed, inspected under light microscopy and replaced in fresh lubricant. At intervals of 500,000 cycles, the cups were removed, cleaned, dried and weighed to indicate the amount of wear. After inspection under light microscopy, the cups were replaced on the wear machine with fresh lubricant and testing was continued to a total of three million cycles. One million cycles is approximately the equivalent of one year's walking activity of a typical patient.
The weight loss was corrected for the effects of fluid absorption (which masks wear) by increasing the apparent weight loss of the wear test cups by the mean weight gain of three control cups of each material that were also immersed in serum and cyclically loaded on a separate frame, but without oscillation. The corrected rate of weight loss was converted to volume loss by dividing by the approximate density of UHMWPE (0.94 gm/cc). The mean weight loss (after soak correction) and the standard deviation was calculated for each of the four types of materials at each weighing interval. The wear rate of each cup was calculated by applying linear regression to the wear data for the entire three million cycles. The mean wear rates and standard deviations also were calculated for each type of material.
Results
The wear of the cups subjected to 3.3 Mrad with remelting averaged 18.6 mm3 per million cycles, or 12% lower wear than for the non-remelted 3.3 Mrad cups. The cups subjected to 28 Mrad had much lower wear rates than the 3.3 Mrad cups, and the rates were similar, whether or not the material had been remelted. That is, the average wear rate of the non-remelted 28 Mrad cups was about 1.2% that of the non-remelted 3.3 Mrad controls, and the average wear rate of the remelted 28 Mrad cups was about 1.7% of the same controls.
Discussion
The results of the wear test clearly demonstrated the improved wear resistance of the UHMWPE acetabular cups that resulted from exposure to 28 Mrad gamma radiation. Apparently, the crosslinking generated by the higher radiation dose reduced the wear rates to less than a few percent or the control value (3.3 Mrad). The minimum amount of wear debris necessary to induce clinically significant osteolysis and other problems in a specific patient has not been established, and it may vary among patients. Nevertheless, a material which reduces the wear rate to the very low levels exhibited by the 28 Mrad cups in this study would be very likely to provide a large margin of safety over currently used materials.
The wear curves for both of the 28 Mrad specimens (
Materials
Six UHMWPE (GUR 4150) extruded bars (3″ diameter) were gamma irradiated in air, three bars each at 3.3 or 28 Mrad, at a dose rate of 0.2 Mrad/hour. For each radiation dose, two bars were then remelted by heating in an oven at ambient atmosphere from room temperature to 150° C. at about 0.3° C./min, holding at 150° C. for 5 hours and slowly cooling to room temperature, and the third bar was not remelted. A 13 mm (0.5 inch) layer of the outer diameter of the treated (remelted) and untreated (non-remelted) bars was machined away to remove the most oxidized, least crosslinked surface layer. The bars were used to produce specimens for the artificial aging tests described here and for the wear tests described in EXAMPLE 2.
To examine the effect of artificial aging on these four materials (3.3 and 28 Mrad, remelted and not remelted), 8 mm thick disks were cut from these 2 inch diameter cores and were heated in an oven slowly (˜0.2° C./min) to 80° C. at ambient atmosphere and held at 80° C. for 10, 20 or 30 days. In addition, one acetabular cup for each of the four conditions (3.3 and 28 Mrad, remelted and not remelted) that had been fabricated at the same time as the wear test cups of EXAMPLE 2 and stored in air for about 5 months was cut into four pieces and aged at 80° C. for the same periods.
The gel content analysis and DSC method are as described in EXAMPLE 1, above.
Fourier Transform Infrared Spectroscopy (FTIR)
FTIR measurements were performed on the above specimens. Segments about 5 mm wide were cut from each polyethylene specimen and the segments were microtomed into 200 μm thick slices. The oxidation profiles, as indicated by the carbonyl concentration, were measured using a Mattson Polaris FTIR (model IR 10410) with a Spectra-Tech IR plan microscope. Spectra were collected in 100 μm steps from the surface to the middle of the specimen, using 64 scans summation at a resolution 16 cm−1 with a MCT (Mercury Cadmium Telluride) detector. The carbonyl group concentration was indicated by the ratio of the peak height of the ketone absorption band at 1717 cm−1 to the height of the reference band at 2022 cm−1 (—CH2-vibration).
Results
The oxidation profiles as a function of depth are shown in
Similarly, with the acetabular cups stored in air for 5 months and then aged for 20 days at 80° C., the remelted materials (3.3 or 28 Mrad) showed no oxidation (
Since crosslinking of UHMWPE reduces its solubility, the percent of undissolved material (gel content) is an indirect indication of the amount of crosslinking. The gel content as a function of depth for various conditions are shown in
The results of the DSC measurements indicated the degree of crystallinity as a function of depth for various materials aged for 30 days at 80° C., as shown in
Discussion
Irradiation of UHMWPE produces crosslinking, chain scission and the formation of free radicals. If oxygen is present, it may react with the free radicals to form oxidized species, leading to additional chain scission (reduction in molecular weight) and an increase in crystallinity. Since polymer crystallizes melt and become amorphous above the melting temperature, molecular chain movements and rotations are increased, favoring the recombination of free radicals. The results of the present experiments showed that remelting at 150° C. apparently caused the residual free radicals to decay and/or to recombine to form crosslinks, leading to an increased gel content. Therefore, remelting is an effective way to extinguish free radicals, making the material less susceptible to long-term oxidation and potentially improving the long-term wear resistance, as evident from the results of the artificial aging experiments, where there was much less oxidation of the remelted materials.
For a crosslinked polymer, oxidative degradation cleaves the molecules and leads to a reduction in gel content. This was evident in the present experiments from the reduced gel content after aging, particularly with the non-remelted materials (
An appropriate amount of crosslinking of UHMWPE can improve its wear resistance. The high level of crosslinking in the UHMWPE caused by the 28 Mrad gamma irradiation, as evident from the high gel content (EXAMPLE 2), apparently contributed to the much greater wear resistance exhibited by the acetabular cups tested in EXAMPLE 2. In addition, as shown in EXAMPLE 3, remelting of the irradiated UHMWPE markedly reduced the residual free radicals, rendering the material much more resistant to subsequent oxidation and, therefore, resistant to a reduction in crosslinking, which can be of substantial benefit for implants in long-term clinical use.
Materials and Methods
The wear testing of irradiated cups with and without remelting was described in EXAMPLE 2. Effects of artificial aging on the physical properties of irradiated UHMWPE, with and without remelting, were described in EXAMPLE 3. To examine the resistance of crosslinked cups to thermal-induced oxidation, and the effect of such oxidation on the wear of irradiated cups with and without remelting, two acetabular cups for each of the four conditions (3.3 and 28 Mrad, remelted and not remelted) that had been wear tested for 3 million cycles as described in EXAMPLE 2, were heated in an oven slowly (˜0.2° C./min) to 80° C. at ambient atmosphere and held at 80° C. for 20 days, with one acetabular cup for each of the four conditions being stored in ambient air. The oxidation profile after 20-day aging for each condition was shown in
Prior to wear testing, the cups were pre-soaked in distilled water for four weeks to minimize additional fluid absorption during the wear test, thereby making the weight loss method for wear measurement more accurate. The details for the wear test were described in EXAMPLE 2.
Results
All cups subjected to 3.3 Mrad with remelting showed comparable wear rates, whether or not the material had been remelted or remelted and aged. Wear was negligible for all of the cups subjected to 28 Mrad, whether or not these were remelted, and whether or not they were aged.
Discussion
The results of the wear test clearly demonstrated the improved wear resistance of the UHMWPE acetabular cups that resulted from exposure to 28 Mrad gamma radiation. Apparently, the minor oxidation at the surface (
For the non-remelted 3.3 Mrad cups, subsurface oxidation, peaking at about 1 mm below the surface (
Nevertheless, the sub-surface oxidation in the non-remelted cups (EXAMPLE 3, particularly for the 28 Mrad specimen) leads to reduced molecular weight, a reduction in crosslinking (as indicated by gel content) and an increased crystallinity and brittleness, all of which can contribute to reductions in mechanical properties such as fatigue strength and, eventually, a reduction in wear resistance. Although remelting had no apparent effect on the wear resistance of the aged cups in the present example, the elimination of free radicals by remelting improves the long-term resistance to oxidation, thereby improving the long-term wear resistance in vivo.
Materials and Methods
In EXAMPLE 2, we demonstrated the improved wear resistance of UHMWPE acetabular cups that resulted from exposure to 28 Mrad gamma radiation, as compared to cups irradiated to 3.3 Mrad. The average wear rate of the 28 Mrad cups was less than 2% of that of the 3.3 Mrad cups (i.e., a dose within the normal 2.5 to 4.0 Mrad range used to sterilize implants). To examine the wear as a function of radiation dose and, thereby, determine an optimum dose for reducing wear, extruded bars of GUR 4150 UHMWPE, 3″ diameter×15″ long, were gamma irradiated in air, three bars at each dose of 4.5, 9.5, 14.5, 20.2 or 24 Mrad (SteriGenics, Inc., Corona, Calif.), at a dose rate of 0.45 Mrad/hour. Additional bars were irradiated in air to 50 or 100 Mrad (SteriGenics Inc., Tustin, Calif.), at a dose rate of 0.67 Mrad/hour. For each radiation dose, two bars were then remelted by heating in an oven in ambient atmosphere from room temperature to 150° C. at about 0.3° C./min, holding at 150° C. for 5 hours and then slowly-cooled to room temperature, with the third bar not being remelted. The irradiated-remelted bars were used to produce acetabular cups for the wear tests.
Seven sets of acetabular cups were machined from the irradiated-remelted bars for each of the seven doses at a commercial machining shop (Bradford and Meneghini Manufacturing Co., Santa Fe Springs, Calif.). Each cup had a 2″ O.D. and 1.26″ I.D., with 1″ outer radius and 0.63″ inner radius (
Because acetabular cups used in patients must first be sterilized by some acceptable means, the test cups in this study were sterilized prior to wear testing using ethylene oxide at the appropriate dose for clinical implants. Ethylene oxide was chosen instead of additional gamma irradiation (e.g., 2.5-4.0 Mrad) in order to focus the results on the effects of the radiation doses used to crosslink the materials. Prior to wear testing, the cups were pre-soaked in distilled water for four weeks to minimize additional fluid absorption during the wear test, thereby making the weight loss method for wear measurement more accurate. The details for the wear test method were described in EXAMPLE 2.
Results
The wear of the cups subjected to 3.3 or 4.5 Mrad with remelting averaged 17.5 or 9.3 mm3 per million cycles, respectively, showing about 13% or 54% lower wear than for the 3.3 Mrad non-remelted cups (20.1 mm3 per million cycles). In contrast, the wear rate of the 9.5 Mrad remelted cups averaged 2.2 mm3 per million cycles, i.e., about 89% lower than for the 3.3 Mrad non-remelted cups. For radiation doses greater than 9.5 Mrad, minimal systematic wear occurred, such that, compared to that with 3.3 Mrad non-remelted cups, the wear rates were about 94% lower for the 14.5 Mrad remelted cups, and minimal wear (>99% reduction) for the 20.2 Mrad remelted cups.
“Negative” wear rates were calculated for the cups given 24 Mrad or greater doses. Apparently, these cups absorbed more water than the soak control cups, and the error between the two was greater than the weight loss due to wear, giving a net gain in weight.
Discussion
The results clearly demonstrated that the wear resistance of UHMWPE acetabular cups were improved substantially with increasing radiation dose over the range of 4.5 to 9.5 Mrad (i.e., with increasing crosslinking), such that wear was too small to accurately quantify for doses exceeding about 20 Mrad. Since, in addition to improving wear resistance, radiation induced crosslinking may degrade other physical properties, such as elongation to failure and fatigue strength, the dose-response curve developed in the present example provides the opportunity to select an optimum dose, i.e., one that provides the desired amount of improvement in wear resistance with a minimum reduction in other physical properties. The procedure for arriving at the choice of dose for a particular in vivo application is described in this application.
UHMWPE acetabular cups that had been compression molded and then exposed to 3.1 Mrad gamma radiation in air but were not thermally treated (i.e., typical of commercially used implants over the past two decades), showed an approximate wear rate of 33.1 mm3/million cycle using the procedure of the wear test described in EXAMPLE 2, above. When compared to these conventional UHMWPE acetabular cups, the acetabular cups of the present invention (i.e., irradiated bar stock, remelted and machined into cups) show the following percentage reduction in wear rate: for the 3.3 Mrad remelted acetabular cup from EXAMPLE 2, above (about 47% reduction in wear rate); 4.5 Mrad remelted acetabular cup from EXAMPLE 5, above (about 72% reduction in wear rate); 9.5 Mrad remelted acetabular cup from EXAMPLE 5, above (about 93% reduction in wear rate).
Materials and Methods
The materials for physical characterization were the same as the wear tested materials described in EXAMPLE 5. The materials included UHMWPE extruded bars (3″ in diameter) gamma irradiated to 3.3, 4.5, 9.5, 14.5, 20.2, 24, 50 and 100 Mrad, with or without remelting, and the non-irradiated bars. 8 mm thick disks were cut out of irradiated bars with or without remelting, and sterilized with ethylene oxide. The specimens for DSC and swelling measurements were cut out of the center of the 8 mm thick disks. The DSC measurement for crystallinity and melting temperature with sample weighing about 4 mg was described in EXAMPLE 1. For swelling measurements, 1 mm thick sheet weighing about 0.5 gram was cut out of the center of the 8 mm thick disk, and extraction of the sol-fraction was performed in boiling p-xylene for 72 hours, with 0.5 wt % antioxidant (2,6-di-t-butyl-4-methyl phenol) being added to prevent oxidation. After extraction, the gel was transferred to fresh p-xylene and allowed to equilibrate at 120° C. for 2 hours. The swollen gel was then quickly transferred to a weighing bottle, covered and weighed. The data was obtained as the average of five measurements. After measurements, samples were deswollen in acetone and then dried at 60° C. in a vacuum oven to a constant weight. The gel fraction was determined as the ratio of the weight of the dried extracted to the initial dry non-extracted network. The degree of swelling was calculated as the ratio of the weight of the swollen gel to the dried extracted gel. The degree of swelling was used to calculate the network chain density, number-average molecular weight between crosslinks and crosslink density, according to the theory of Flory and Rehner {Shen et al., J. Polym. Sci., Polym. Phys., 34:1063-1077 (1996)}. For examining the oxidation profiles of the extruded bars irradiated and remelted in air, a two hundred micron thick section was microtomed perpendicular to the bar surface and examined by FTIR as a function of depth from the bar surface.
Results and Discussion
The melting temperature and crystallinity for non-irradiated, and irradiated (with and without remelting) materials are shown in Table 6. The degree of swelling, average molecular weight between crosslinks, crosslink density and gel content are shown in Table 7. After irradiation, the melting temperature and crystallinity increased, ranging from 135.3 to 140.2° C., and about 60 to 71%, respectively, over the dose range studied. Remelting of the irradiated bars resulted in reductions in the melting temperature and crystallinity, ranging from about 131 to 135° C., and about 51 to 53%, respectively.
As shown in Table 7, with increasing radiation dose, the degree of swelling and average molecular weight between crosslinks decreased, while the crosslink density increased. The gel content, in general, increased with radiation dose, but reached a plateau region at about 9.5 Mrad. With remelting, the degree of swelling and average molecular weight between crosslinks for bars irradiated up to 9.5 Mrad were significantly reduced, but remained almost unchanged after 9.5 Mrad. The crosslink density increased, after remelting, with dose up to 9.5 Mrad and then remained almost unchanged. The gel content, generally, increased after remelting.
The oxidation profiles for the 9.5 and 24 Mrad materials, after remelting at 150° C. in air for 5 hours, as a function of depth from the bar surface are shown in
Materials and Methods
The materials for tensile test are the same as the wear tested materials described in EXAMPLE 5, above. The materials included UHMWPE extruded bars (3″ in diameter) gamma irradiated to 4.5, 9.5, 14.5, 20.2, and 24 Mrad, with or without remelting, and non-irradiated bars. Five tensile specimens each was machined out of the center of the 3″ diameter bars according to ASTM F648-96 and D-638 (type IV). Tensile tests were performed using an servo-hydraulic tensile test machine at speed of 2 inches/min.
Results and Discussion
The tensile strength at yield, elongation, and tensile strength (ultimate) at breaks are shown in Table 8. The average tensile properties as a function of radiation dose are shown in
All publications and patent applications mentioned in this Specification are herein incorporated by reference to the same extent as if each of them had been individually indicated to be incorporated by reference.
Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity and understanding, it will be obvious that various modifications and changes which are within the skill of those skilled in the art are considered to fall within the scope of the appended claims. Future technological advancements which allows for obvious changes in the basic invention herein are also within the claims.
TABLE 1
3.3 Mrad
Before remelting
After remelting
Distance
peak melting
degree of
peak melting
degree of
from surface
temperature
crystallinity
gel content
temperature
crystallinity
gel content
(mm)
(° C.)
(%)
(%)
(° C.)
(%)
(%)
34.8-35
135.3
59.6
91
131.4
52.5
94.7
35.8-36
135.4
60.2
91
131.5
51.2
94.7
36.8-37
135.3
60.5
91
131.4
51.9
94.7
37.8-38
135.3
60
91.1
131.3
52
95
(center)
TABLE 2
28 Mrad
Before remelting
After remelting
Distance
peak melting
degree of
peak melting
degree of
from surface
temperature
crystallinity
gel content
temperature
crystallinity
gel content
(mm)
(° C.)
(%)
(%)
(° C.)
(%)
(%)
34.8-35
139.8
65.1
95.8
135
52
97.7
35.8-36
139.8
64.2
95.8
134.8
52.1
97.7
36.8-37
139.7
64.5
95.8
134.9
52.5
97.7
37.8-38
139.7
65.3
95.8
134.9
52.7
97.7
(center)
TABLE 3
Wear Rate
Mean Wear
(mm3/million
Rate ± Std
Cup #
Material
cycles)
Deviation
N11
3.3 Mrad
20.8
21.1 ± 0.3
N16
Not
21.2
N17
remelted
21.4
R21
3.3 Mrad
17.7
18.6 ± 1.3
R26
Remelted
20.1
R27
18.0
N35
28 Mrad
0.29
0.25 ± 0.03
N31
Not
0.24
N32
remelted
0.24
R48
28 Mrad
0.36
0.36 ± 0.001
R45
Remelted
0.35
R49
0.36
TABLE 4
0-3 Million Cycles
(non-aged)
3-7 Million Cycles
Wear Rate
Mean Wear
Wear Rate
Mean Wear
(mm3/million
Rate ± Std
(mm3/million
Rate ± Std
Cup #
Material
cycles)
Deviation
Conditions
cycles)
Deviation
N11
3.3 Mrad
20.8
21.1 ± 0.3
non-aged
21.2
—
N16
Not
21.2
aged
21.5
21.8 ± 0.5
N17
remelted
21.4
aged
22.2
R21
3.3 Mrad
17.7
18.6 ± 1.3
non-aged
17.5
—
R26
Remelted
20.1
aged
19.2
19.8 ± 1.0
R27
18.0
aged
20.5
N35
28 Mrad
0.29
0.25 ± 0.03
non-aged
0.03
—
N31
Not
0.24
aged
−0.47
−0.71 ± 0.3
N32
remelted
0.24
aged
−0.93
R48
28 Mrad
0.36
0.36 ± 0.001
non-aged
0.47
—
R45
Remelted
0.35
aged
0.08
−0.06 ± 0.2
R49
0.36
aged
−0.20
TABLE 5
(1-5 million cycles)
Wear Rate
Mean Wear
(mm3/
Rate ± SD (mm3/
Cup #
Material
million cycles)
million cycles)
N11
3.3 Mrad Not remelted
20.46
20.12 ± 0.7*
N16
19.32
N17
20.59
R21
3.3 Mrad Remelted
17.04
17.51 ± 0.48*
R26
18.0
R27
17.49
RA2
4.5 Mrad Remelted
9.93
9.28 ± 0.92
RA3
8.63
RB3
9.5 Mrad Remelted
2.39
2.22 ± 0.24
RB6
2.05
RC5
14.5 Mrad Remelted
1.26
1.17 ± 0.13
RC6
1.08
RD1
20.2 Mrad Remelted
0.26
0.12 ± 0.2
RD6
−0.02
RE3
24 Mrad Remelted
−0.49
−0.59 ± 0.13
RE4
−0.68
RF2
50 Mrad Remelted
−0.8
—
RG1
100 Mrad Remelted
−6.88**
—
*The wear data of the 3.3 Mrad materials in Example 2.
**The wear rate in the period of 1-2 million cycles.
TABLE 6
Non-remelted
Remelted
Melting
Crystallinity
Melting point
Crystallinity
Samples
point (° C.)
(%)
(° C.)
(%)
Non-irrad.
133.8
55
—
—
3.3 Mrad
135.3 ± 0.1
60.1 ± 0.4
131.4 ± 0.1
51.8 ± 0.6
4.5 Mrad
136.2 ± 0.2
65.8 ± 1.6
131.6 ± 0.2
52.0 ± 1.3
9.5 Mrad
137.1 ± 0
67.1 ± 2.2
134.8 ± 0.2
53.3 ± 2.1
14.5 Mrad
137.5 ± 0.2
69.6 ± 1.6
135.0 ± 0.1
53.0 ± 1.5
20.2 Mrad
137.4 ± 0.1
70.8 ± 2.8
135.3 ± 0.1
52.1 ± 1.8
24 Mrad
137.9 ± 0.3
68.0 ± 1.3
135.2 ± 0.1
51.7 ± 1.2
50 Mrad
138.9 ± 0.2
67.0 ± 1.3
135.2 ± 0
52.8 ± 0.2
100 Mrad
140.2 ± 0.3
66.3 ± 2.7
130.8 ± 0.2
52.3 ± 1.7
TABLE 7
Non-remelted
Remelted
M.W.
M.W.
between
Crosslink
Gel
between
Crosslink
Gel
Degree of
crosslinks
density
content
Degree of
crosslinks
density
content
Samples
swelling
(g/mol)
(mol %)
(%)
swelling
(g/mol)
(mol %)
(%)
3.3 Mrad
5.29
8400
0.17
94.7
3.21
2500
0.56
98.1
4.5 Mrad
3.57
3500
0.40
97.8
3.15
2400
0.58
98.4
9.5 Mrad
2.82
1900
0.74
98.6
2.54
1400
1.0
98.9
14.5 Mrad
2.35
1100
1.27
98.7
2.36
1100
1.27
99.2
20.2 Mrad
2.27
1000
1.40
98.8
2.25
1000
1.40
99.2
24 Mrad
2.17
900
1.56
98.7
2.24
1000
1.40
99.2
50 Mrad
1.92
600
2.33
98.7
2.37
900
1.56
99.1
100 Mrad
1.71
400
3.50
98.6
1.71
400
3.50
98.5
TABLE 8
Tensile Strength at
Tensile Strength at
Elongation at
Materials
Yield (MPa)
Break (MPa)
Break (%)
Non-irradiated
23.3 ± 0.11
52.1 ± 4.78
356 ± 23
Without remelting
4.5 Mrad
24.9 ± 0.33
46.9 ± 2.91
314 ± 12
9.5 Mrad
25.3 ± 0.12
47.6 ± 2.76
251 ± 8
14.5 Mrad
25.7 ± 0.25
46.4 ± 1.20
213 ± 5
20.2 Mrad
26.2 ± 0.27
40.2 ± 2.72
175 ± 7
24 Mrad
26.4 ± 0.23
40.0 ± 5.42
164 ± 17
After remelting
4.5 Mrad
21.5 ± 0.33
45.6 ± 8.89
309 ± 20
9.5 Mrad
21.3 ± 0.60
43.2 ± 2.80
252 ± 8
14.5 Mrad
21.8 ± 0.29
36.8 ± 1.72
206 ± 9
20.2 Mrad
21.9 ± 0.18
34.3 ± 3.61
185 ± 8
24 Mrad
21.7 ± 0.25
32.3 ± 2.81
160 ± 19
Shen, Fu-Wen, McKellop, Harry A., Salovey, Ronald
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